This invention relates to a transformer for a current sensor.
The transformer here is to be understood as a magnetic configuration in the general sense, which is magnetically coupled to a primary conductor and electrically coupled to an electronic analyzer. This forms a current sensor, which makes it possible to determine the current in the primary conductor, both AC and DC current detection being equally possible.
For example, so-called compensation current sensors are known; they have a toroidal magnetic core with an air gap. The magnetic core is provided with a secondary winding (compensation winding). The primary conductor, i.e.; the conductor carrying the current to be measured, passes centrally through the magnetic core. A magnetic field probe which detects the flux in the magnetic core is situated in the air gap of the core. The probe signal is sent via an amplifier circuit to the secondary winding (compensation winding) such that the magnetic flux induced by the primary conductor is constantly being compensated to zero. The secondary current required for this is then strictly proportional to the primary current to be measured. Based on a linear primary conductor, the proportionality factor is determined based on the number of windings of the secondary winding (compensation winding). A probe that operates according to the Hall principle, in particular a corresponding integrated circuit, is often used as the magnetic field sensor. In the case of such a Hall probe, the air gap is absolutely essential because the magnetic field sensitivity is relatively low. The field concentration due to the magnetic core is thus necessary to ensure a sufficiently high sensitivity of the control circuit.
Use of a Hall probe has the disadvantage of a definite offset, which means that without the applied magnetic field, the output voltage is different from zero. Furthermore, this offset has a marked temperature dependence, which results in a definite offset having a marked temperature dependence [sic].
In addition, compensation current sensors having a field probe made of a soft magnetic material are known, such as those described in European Patent Application 0 294 590 A2, for example. A closed magnetic core is used in this embodiment. In addition to the magnetic core, at least one strip-shaped element made of an amorphous soft magnetic material equipped with an indicator winding is arranged next to the magnetic core. This element then functions as the magnetic field probe. For analysis, a pulse-shaped bipolar magnetization is induced with the help of an indicator winding, and the asymmetry of the current and/or voltage amplitudes is used to determine and analyze the mismatch. This arrangement has the advantage over the configuration using the Hall probe that the hysteresis of this probe is negligible. In the case of a compensation current sensor, this results in a very small offset. In addition, the temperature drift of such a probe is negligible in most cases, so the offset is also thermally stable.
However, the arrangement mentioned last has the disadvantage that the magnetic field probe detects the field of the primary conductor, i.e.; the field in the atmosphere. There is no concentration of flux due to the magnetic core. Since the magnetic field probe is a strip-shaped element, the effective permeability of the magnetic core of the probe is relatively low due to the strong shearing of the magnetic circuit. On the whole, this results in a moderate sensitivity of the magnetic field probe. However, a low sensitivity of the magnetic field probe results in a low magnification in the first step of the control circuit, so that an offset of the participating amplifiers is manifested as an offset of the entire current sensor.
To avoid this disadvantage, an air gap is introduced into the magnetic core in European Patent Application 0 294 590 A2. This air gap ensures a concentration of flux at the probe location and thus results in a greater sensitivity of the entire arrangement. This eliminates the first disadvantage of the arrangement.
However, a second disadvantage of this arrangement involves a certain external field sensitivity. The magnetic field probe(s) can also detect external magnetic fields just like the field of the primary conductor and that of the compensation coil. When using two magnetic field probes, an external field is compensated because the probes are oppositely polarized. However, this is true only in a completely homogeneous external magnetic field, but in reality such a magnetic field exists only in extremely rare cases, because external magnetic fields emanate from adjacent current conductors or transformers, for example, which emit a strongly heterogeneous magnetic field.
A third disadvantage of the configuration disclosed in European Patent Application 0 294 590 A2 is the dependence on the position of the primary conductor. This configuration has a high precision only when an idealized, infinitely long conductor passes through the magnetic core exactly on its axis. Only then are the probes flooded by the magnetic core precisely so as to yield a strict proportionality between the secondary current and the primary current. For example, if the primary conductor is passed as a loop over a ring segment of the magnetic core, a very high flux is fed into the magnetic core at this location, while the H field at the site of the probes is definitely lower than in the axial bushing of the primary conductor. Although the sensor will still supply an output signal having a linear current in this case, its slope is much lower than in the optimum design, and its measurement range also turns out to be much lower because of saturation effects in the magnetic core.
These disadvantages are compensated by additional measures described in the related art. These measures include the special winding of the sensor strip (as described in European Patent 0 538 578), the special shape of the stacked magnetic core (as disclosed in European Patent 0 510 376), the use of a bending core (as disclosed in German Patent 197 20 010.9) and a tight coupling between the primary side and the secondary side (corresponding to German Patent 197 05 770.8, for example).
Based on the configuration described in European Patent Application EP 0 294 590 A2, only a single magnetic field probe is assumed for the following discussion. The air gap is retained to ensure an adequate sensitivity of the magnetic field probe. However, the magnetic field probe is introduced into the air gap of the magnetic core and/or into a pocket of the magnetic core. This greatly reduces the external field sensitivity because the magnetic core has a shielding effect. In addition, this avoids a dependence on the position of the primary conductor because any flux fed into the magnetic core by the primary conductor must penetrate through the magnetic field probe. These state-of-the-art current sensors, which can we manufactured inexpensively, are thus characterized by the following properties:
1. They have a much smaller offset than comparable current sensors using Hall magnetic field probes because the magnetic probe does not have any hysteresis.
2. They have a lower external field sensitivity.
3. They have only a very low dependence on the position of the primary conductor.
4. The sensor can be manufactured very inexpensively because the magnetic core consists of two punched blanks/bent parts and the compensation coil consists of a simple spool winding.
On the other hand, however, disadvantages are associated with such embodiments. For example, the magnetic field probe sits in the air gap of the magnetic core and is thus influenced by the remanence of the magnetic core. Magnetic cores made of nickel/iron material that can be manufactured inexpensively have a certain coercitive field strength, which results in a remanent magnetic flux even without any external control. The ends of the magnetic core in the air gap act like pole faces, i.e.; even without control here there is a certain field which influences the magnetic field probe. This results in an offset of the current sensor. This offset is much smaller than that with current sensors using Hall magnetic field probes, but in certain applications it may still be manifested as interference. With respect to the primary current, it is approximately of the order of magnitude of 50 to 100 mA. Such an offset means a lower limit to the measurement range because currents on the order of magnitude of the offset can no longer be detected accurately.
Another disadvantage of the known arrangement is derived from the fact that the secondary coil is applied to only one sector of the magnetic core in the area of the magnetic field probe. The magnetic flux of the secondary coil is not closed exclusively via the magnetic core but instead also as stray flux in space. This stray flux is missing in the areas of the magnetic core outside of the compensation coil for compensation of the flux induced by the primary conductor. Outside of the compensation coil, the magnetic core is therefore triggered in proportion to the intensity of the primary current. Beyond a certain primary current, the saturation flux density of the magnetic core is reached and the magnetic core is partially saturated. Beyond this point, the current sensor is highly nonlinear, i.e.; this effect forms the upper limit of the measurement range of the current sensor.
Therefore, the object of this invention is to provide a current sensor which will avoid the saturation problems described above while also having an extremely small offset.
This object is achieved by a transformer according to Patent Claim 1. Embodiments and refinements of the idea of this invention are the object of the subclaims.
Transformers according to this invention are characterized by a very large measurement range, but current sensors designed with them are nevertheless compact and are simple and easy to manufacture from the standpoint of their structural embodiment. In particular, they have a smaller offset than comparable current sensors using Hall magnetic field probes, because the magnetic field probe does not have any hysteresis. They also have a lower external field sensitivity and have only a low dependence on the position of the primary conductor. Finally, this current sensor can be manufactured very inexpensively because the magnetic core may consist of two punched blanks/bending parts and the compensation coil may consist of a simple spool winding.
This is achieved in particular by a transformer having a closed probe core made of a soft magnetic material, a probe winding, which is wound around the probe core in at least some sections, a closed compensation core made of a soft magnetic material and a compensation winding which is wound around the probe core and the compensation core in at least some sections, the probe core and the compensation core being arranged relative to one another such that a conductor carrying the current to be measured can be passed through the probe core and the compensation core. The probe winding and/or the compensation winding may be wound only in part or preferably completely around the complete extent of the core. Due to the complete winding, a symmetrical distribution of the magnetic field with a low leakage field is achieved.
In addition, an amorphous metal, i.e.; a nanocrystalline metal, may be used as the soft magnetic material, preferably in the case of the probe core but also in the case of the compensation core or both.
The probe core and the compensation core are preferably designed as (round) ring cores, the probe core and the compensation core being arranged concentrically relative to one another. In addition, it is possible to provide for the probe core to have a considerably smaller cross-sectional area than the compensation core.
In a refinement of this invention another closed compensation core is provided, the probe winding being wound in at least some sections only around the probe core and the compensation winding being wound in at least some sections around the probe core and both compensation cores. The probe core is preferably situated between the two compensation cores. In this way, optimum shielding of the compensation core and the compensation winding and thus extensive shielding from interference fields are achieved.